1. Field of the Invention
The present invention relates to a motor shaft, a motor and a motor assembly.
2. Description of the Related Art
A method of fixing a press-fitted component such as a gear to a motor shaft includes a method of forming a hole for inserting a motor shaft into a press-fitted component, processing the hole such that the inner diameter of the hole of the press-fitted component is slightly less than the outer diameter of the motor shaft and press-fitting and fixing the motor shaft into the hole of the press-fitted component.
Power required to press-fit the motor shaft in the press-fitted component is approximately proportional to a value obtained by multiplying a press-fitting interference ((the outer diameter of the motor shaft)−(the inner diameter of the press-fitted component), by a press-fitting length. If power required for press-fitting is greater than the allowable compression stress of the motor shaft, when the motor shaft is press-fitted into the press-fitted component, the motor shaft may buckle. By decreasing the press-fitting length, power required for press-fitting may decrease such that the motor shaft does not buckle. However, when the press-fitting length decreases, when the motor shaft is press-fitted into the press-fitted component, an inclination easily occurs in the axis of the motor shaft and the axis of the press-fitted component. Therefore, it is not easy to fix the press-fitted component to the motor shaft with high precision.
When the press-fitting length is large, it is possible to fix the press-fitted component to the motor shaft with high precision. When the press-fitting length is high, it is necessary to decrease the upper limit of the press-fitting interference such that power required for press-fitting is less than the allowable compression stress of the motor shaft. The tolerance of the outer diameter of the motor shaft and the inner diameter of the hole of the press-fitted component may be decided according to design so as to decrease the upper limit of the press-fitting interference. However, in an actual manufacturing process, it may be difficult to control the outer diameter of the motor shaft and the inner diameter of the press-fitted component according to design. Therefore, adjustment in a manufacturing process such as stratifying of the processed motor shaft and press-fitted component is required such that power required for press-fitting becomes less than the allowable compression stress of the motor shaft. Accordingly, there is a need for a motor shaft which may be press-fitted into a press-fitted component with small power such that the motor shaft does not buckle.
A serration-processed motor shaft capable of being press-fitted into a female part with small power is suggested in JP-A-2012-257389.
However, even in the motor shaft suggested in JP-A-2012-257389, power required for press-fitting increases in proportion to the press-fitting length. When the press-fitting length increases, adjustment in a manufacturing process such as stratifying of the processed motor shaft and press-fitted component is required such that power required for press-fitting becomes less than the allowable compression stress of the motor shaft.